Exemplary embodiments of the invention relate to a method for monitoring a screw centrifuge.
The screw centrifuge to be monitored may have been configured as a solid-bowl screw centrifuge, or as a screen-bowl screw centrifuge, for instance.
EP 0 798 046 A1 discloses a centrifuge drive with two motors—a primary motor and a variable-speed motor—and with a three-stage transmission. On three shafts a torque is either introduced into the transmission or picked up from it.
DE 10 2006 028 804 A1 discloses a screw centrifuge with a centrifuge drive with two motors—a primary motor and a variable-speed motor—and with a three-stage transmission. In this case, torques are capable of being introduced into the first transmission stage and the second transmission stage or capable of being picked up from these two transmission stages on a total of at least four shafts, in which connection, furthermore, the first and second transmission stages are particularly preferably capable of being driven (and, as a rule, are also driven) on at least three shafts, the first motor feeding a torque both into the housing and into the first transmission stage on two shafts.
In the course of the operation of a screw centrifuge, the difference in rotational speed between the drum and the screw is set by the mechanism of the machine or by the actuation of a variable-speed motor. However, in some applications of the screw centrifuge—that is to say, in the course of a separation of a product from solids conveyed out of the drum by the screw—under, for the most part, indeterminate operating conditions the so-called “stick-slip effect” may arise. This stick-slip effect between drum and screw is associated with strong surges of torque (sometimes even with fluctuating directions of torque), which load the drivetrain and, under certain circumstances, may even lead to damage and to stoppage of the plant if they are not noticed in time.
In contrast, exemplary embodiments of the invention are directed to a method for monitoring a screw centrifuge, with which an onset of a stick-slip effect can be ascertained in good time.
According to an embodiment, a method for monitoring a screw centrifuge, in particular a solid-bowl centrifuge or a screen-bowl centrifuge, including the following: a rotatable drum, a rotatable screw arranged in the drum, a main or primary motor at least for driving the drum, a drive motor for driving the screw (within the scope of this document, this also means, in particular, influencing the difference in rotational speed of the screw relative to the drum), which may be the main or primary motor or a secondary motor, and also a transmission arranged between the motor or motors and the drum and screw, transmission input shafts for the main motor and the drive motor for the screw, wherein one or more pulse-generators are arranged at least on the transmission input shaft for the screw, to each of which a proximity-sensor is assigned, with the following steps:
a) providing the screw centrifuge, and processing of a product with the screw centrifuge, in the course of which the product is separated from solids which are conveyed out of the drum by the screw,
b) (repeated, continual) determining a current angular velocity and determination of a mean angular velocity of the transmission input shaft for the screw over time,
c) (repeated) evaluating the measurements from step b), and
d) outputting a warning signal and/or variation of one or more operating parameters of the screw centrifuge, to the extent that dynamic changes of angular velocity are ascertained in the course of the evaluation.
In step b), the measurements of the current angular velocity of a last period—for example, the last 10 seconds—are, for example, averaged, and the mean value is continually updated to this extent. In addition, the current angular velocity is determined, and changes of this value in comparison with the mean value are then registered. Dynamic changes are, in particular, periodic changes.
In this regard, a reference time for one revolution is preferentially ascertained in the no-load state, and deviations from this time are converted into corresponding angles in the course of the subsequent measurements.
According to another embodiment, there is a method for monitoring a screw centrifuge, in particular a solid-bowl screw centrifuge or a screen-bowl screw centrifuge, which includes the following: a rotatable drum, a rotatable screw arranged in the drum, a primary motor for driving the drum and a secondary rotor for driving the screw, and also a transmission, arranged between the motors and the drum and screw, with transmission input shafts for the primary motor and the secondary motor, an elastic element between an output shaft of the secondary motor and the transmission input shaft for the secondary motor, wherein pulse-generators are arranged on the output shaft of the secondary motor and on the transmission input shaft on both sides of the elastic element, to each of which there are assigned proximity-sensors, with the following steps:
a) providing the screw centrifuge, and processing of a product with the screw centrifuge, in the course of which the product is separated from solids which are conveyed out of the drum by the screw,
b) (repeated) measuring a relative angular offset over time between the output shaft and the transmission input shaft on both sides of the elastic element connecting these shafts,
c) (repeated) evaluating the measurements from step b), and
d) outputting a warning signal and/or variation of one or more operating parameters of the screw centrifuge, to the extent that dynamic changes—that is to say, changes that are not constant over a predeterminable time-interval—of angular offset are ascertained in the course of the evaluation. In this regard, dynamic changes are, in particular, periodic changes.
In accordance with the invention, according to variants of the methods described above, it is possible in each instance to notice the incipient onset of the stick-slip effect in good time. This makes it possible to output a warning, to find a better operating point by procedural measures or by a variation of settings—such as a variation of the difference in rotational speed, of the rotational speed of the drum, or of the product feed quantity—or, in case this is necessary for the purpose of protecting the machine, to switch off the decanter.
The elastic element is preferentially an elastic coupling. But the elastic element may also be constituted by a drive belt if a belt drive has been provided between the output shaft and the transmission input shaft for the secondary motor.
A torque-dependent twist-angle of the coupling (or of the belt drive) between the secondary motor and the transmission input shaft on both sides of the elastic element is preferentially measured with high temporal resolution, and harmonic changes of this angle are detected. By virtue of this measurement or these measurements, the stick-slip effect can be detected particularly well in good time. In the case of a belt drive, where appropriate a gear ratio is incorporated correspondingly into the ascertainment.
In this case it is advantageous, and advantageous for the purpose of ensuring a good measurement, if the pulse-generators on the two shafts have been arranged in a fixed angular relationship, for instance with a phase shift—that is to say, with a corresponding angular offset—preferentially with a phase shift between 0° and 360°, and if the pulse-generators have been configured in such a manner that in the course of rotations of the output shaft one pulse or two or more pulses of the pulse-generators are sensed per revolution. Especially in the latter case, a measurement result can be achieved that can be evaluated particularly well.
The output signals of the proximity-sensors are preferentially read out or registered at a high sampling-rate or sampling-frequency by the control device, which with a suitable software measuring program, constitutes a measuring system, the sampling-rate being greater, preferentially several times greater, than the frequency of revolution of the transmission input shaft. Accordingly, it is expedient if the sampling-rate for a screw speed between 1000 revolutions/min and 10,000 revolutions/min amounts to between 2.5 kHz and 250 kHz. The stiffer the elastic element—in particular, the elastic coupling—has been designed to be, the higher the resolution of the measurement of the angle of torsion is intended to be. For this purpose, a correspondingly high sampling-rate of the sensors is required. Theoretically, eigenfrequencies may in fact perturb the measuring process. It is therefore preferred that no eigenfrequencies of the elastic element lie within the range of measurement of the system, since otherwise eigenfrequencies possibly cannot be distinguished, or can only be distinguished with difficulty, from oscillations by reason of stick-slip effects. But should an eigenfrequency of the elastic element nevertheless lie within the range of measurement and be excited/measured, in principle this is not problematic. For in this case it is possible to change one or more control parameters for actuating the centrifuge appropriately, in order to get out of the eigenfrequency range.
In the course of the further evaluation of the measurements, it is advantageous to ascertain a dynamic change of the angular offset between the output shaft and the transmission input shaft based on the result of a mathematical transformation method, in particular by means of a fast Fourier transformation method, since such a method permits the developing stick-slip effect to be detected particularly well in good time.